1. Institute of Crystallography, Russian Academy of Sciences, Leninsky pr. 59, 117333 Moscow, Russia, Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States, Department of Biology, Indiana University, 1001 East Third Street, Bloomington, Indiana 47405, United States, and EMBL, Hamburg Outstation, Notkestrasse 85, D-22603 Hamburg, Germany